The present invention relates to a printing process, such as a keyless lithographic printing process and, in particular, to a device for circulating ink in a printing fluid pan in a printing press.
In the art and practice of high-speed lithographic offset printing, for example, ink is more-or-less continuously conveyed from a suitable reservoir by means of a series of coextensive rollers to a planographic printing plate where the image portions of the printing plate accept ink from the last of the series of inking rollers. A portion of that ink is then transferred to a printing blanket as a reverse image from which a portion of the ink is transferred in the form of a right-reading image to paper or another suitable substrate. It is essential in conventional lithographic printing processes that dampening water containing proprietary additives also be conveyed more-or-less continuously to the printing plate where, by transferring in part to the non-image areas of the printing plate, the water operates to keep those non-image areas free of ink.
In some prior art printing press systems, both the ink and the dampening water are continuously and separately made available to all parts of the printing plate, image and non-image areas alike; and in the absence of dampening water, the printing plate will accept ink in both the image and non-image areas of its surface.
Lithographic printing plate surfaces in the absence of imaging materials have minute interstices and an overall hydrophilic or water-loving character that enhance retention of water rather than ink in the non-imaged areas. Imaging this hydrophilic plate surface creates oleophilic areas according to the desired image format. Subsequently, when water is presented to the inked, imaged plate in appropriate amounts only that ink residing in non-image areas becomes debonded. In its simplest view, this action accounts for the continuous image and non-image differentiation at the printing plate surface which differentiation is essential and integral to the lithographic printing process.
Newspaper printing configurations are known which rely on the inking train of rollers to carry dampening water to the printing plate. Configurations such as those noted above will, together with appropriate ink and dampening concentrate selections, function such that the ink, referred to as a printing fluid, itself carries all of the required dampening water to the printing plate, yet the press functions and is controlled more-or-less conventionally from the viewpoint of lithographic printing.
Planographic printing systems and elements thereof which do not require dampening water, and may therefore be termed single-fluid systems, are known in the prior art. Such systems rely in one way or another on low-surface-energy silicone non-image portions of the printing plate disallowing ink adhesion, thereby forming the basis for differentiation between ink-receptive nonsilicone image areas and of non-ink-receptive non-image silicone areas of the printing plate. Only ink needs to be available to the plate, dampening solutions being unnecessary.
In the printing press systems described above it is typical to provide a first roller, referred to as a pickup roller in the train of inking rollers which is partially submerged in a pan containing the ink or printing fluid. As the pickup roller rotates, its surface picks up the printing fluid from the pan which in turn is transferred along the train of inking rollers. It is well known in the prior art that it is necessary to circulate the ink within the pan past the surface of the pickup roller in order for an even film of ink to be picked up by the surface of the pickup roller. Prior art devices, generally referred to as mixers or agitators, have been used to circulate the ink within the pan. However, such prior art mixers or agitators have met with limited success in circulating the ink without uneven flow past the surface of the pickup roller. In order to overcome this problem prior art circulating pump systems were utilized in which a pump and possibly a reservoir are connected to the pan, and printing fluid is pumped through the pan in order to circulate it. Such ink pans are typically very tall having sloping sides that enabled gravity to gradually cause the viscous inks to flow toward the sump and maintain positive supply of ink to the circulating pump inlet. The pump then, in addition to draining the pan, homogenizes the reservoir ink and returns it as a fresh supply to the pan. Such constructions however, significantly increased the size of printing presses wherein multiple printing couples are combined in one press structure.
The present invention overcomes these drawbacks of the prior art and provides a novel circulating device which produces an even and constant flow of ink past the surface of the pickup roller and which is compact in size, thereby reducing the overall size configuration of the printing press in which it is used. The present invention also eliminates external recirculating pumps and associated piping in prior art keyless printing presses.